The present invention relates to ion implanters and methods for controlling the same.
Ion implanters have been used in a process for manufacturing semiconductor integrated circuits and are able to implant impurities into finely defined regions on a surface of a semiconductor substrate with good accuracy. In the ion implanters described above, an implanting amount (dosage) of impurity ions per unit area is generally obtained by conversion of an ion beam current I detected by a Faraday cup provided at the rear side of wafers. That is, the number N of ions implanted per unit time is represented by the following equation (1).N=I/q(namely, I=q·N)  (1)
In this equation (1), q indicates the charge per one ion.
In actual ion implantation, by collision of ions with gas atoms (or molecules) remaining inside a vacuum chamber, the change in charge may occur in some cases. When the change in charge occurs, even when the number N of implanting ions is not changed, the q is changed, and as a result, the ion beam current I detected by a Faraday cup is also changed. In general ion implantation, since the dosage is controlled on the assumption that q is constant, when the change in charge occurs, the control of the dosage becomes incorrect.
For example, in Japanese Patent Application Publications (JP-A) Nos. 2000-11942 (Document 1) and 10-226880 (Document 2), an ion implanter has been disclosed which is provided with a function of compensating for the influence caused by the change in charge as described above. The ion implanter disclosed in the above documents is provided with a mechanism called pressure compensation (hereinafter referred to as “Pcomp”) mechanism.
In the Pcomp mechanism, from a current detected by a Faraday cup and a pressure detected in the vicinity of a position at which wafers are irradiated with ion beam, a current is calculated which is to be detected when the change in charge does not occur. This current is a current to be detected when the pressure in the vicinity of the position at which the wafers are irradiated with the ion beam is sufficiently low and is hereinafter called a “real beam current I0”. In addition, the current detected by a Faraday cup is hereinafter called a “measured beam current Im” The pressure detected in the vicinity of the position at which the wafers are irradiated with the ion beam is hereinafter called a “measured pressure Pmeasured”.
Among the measured beam current Im, the real beam current I0, and the measured pressure Pmeasured, the following equation (2) is assumed to hold.Im=I0·f(P)  (2)
In the equation (2), f(P) indicates the function of the measured pressure Pmeasured.
In preliminary ion implantation, under the condition in which the ion beam is in a sufficiently stable state, when the beam current Im is measured while the pressure P is being changed, the function f(P) can be obtained from the following equation (3). The condition in which the ion beam is in a sufficiently stable state is the condition in which the real beam current I0 can be regarded as constant.f(P)=Im/I0  (3)
In a graph in which the vertical axis represents the measured beam current Im, and the transverse axis represents the measured pressure Pmeasured, the real beam current I0 is the intercept of the vertical axis (measured beam current Im) when Pmeasured=0 holds and is assumed to be constant.
In actual ion implantation for production, by the use of the function f(P) represented by the above equation (3), a corrected beam current IPcomp is calculated in accordance with the following equation (4-1) using a detected measured beam current Im Subsequently, based on the corrected measured beam current IPcomp thus calculated, the control of dosage is carried out.IPcomp=Im/f(P)  (4-1)
Hence, as the number N of implanting ions, the following equation (4-2) is obtained from the above equation (1).N=IPcomp/q  (4-2)
Hereinafter, the ion implanter disclosed in the above document 2 will be described. This ion implanter has a Pcomp mechanism that compensates for the influence of the change in charge caused by the interaction between implanting ions and gas atoms (or molecules) remaining inside a vacuum chamber. In this Pcomp mechanism, the measured beam current Im is divided by the pressure function f(P) in accordance with the following equation (5), so that the real beam current I0 is calculated which is to be detected when the change in charge does not occur.I0=Im/f(P)  (5)
Based on theoretical handling in document 2, when the change in charge is assumed that the neutralization only occurs from a monovalent state to a neutral state, in ion implantation by a high current ion implanter, the function f(P) can be formulized by the following equation (6). The ion implantation by a high current ion implanter indicates monovalent ion implantation performed at an energy of 180 KeV or less.                                                                         f                ⁡                                  (                  P                  )                                            =                                                                    exp                    ⁡                                          (                                                                        -                                                      K                            1                                                                          ·                                                  P                          1                                                                    )                                                        ·                                      exp                    ⁡                                          (                                                                        -                                                      K                            2                                                                          ·                                                  P                          2                                                                    )                                                        ·                                      exp                    ⁡                                          (                                                                        -                                                      K                            3                                                                          ·                                                  P                          3                                                                    )                                                                      ⁢                                                                  ⁢                ⋯                                                                                        =                              ∏                                  exp                  ⁡                                      (                                                                  -                                                  K                          i                                                                    ·                                              P                        i                                                              )                                                                                                                          =                              exp                ⁢                                  {                                      Σ                    ⁡                                          (                                                                        -                                                      K                            i                                                                          ·                                                  P                          i                                                                    )                                                        }                                                                                        (        6        )            
In the above equation, P1, P2, P3, . . . indicate partial pressures of plurality of gas species present inside the vacuum chamber and are obtained by the following equation (7) with respect to a total pressure PAL.PAL=ΣPi  (7)
In addition, K1, K2, K3, . . . are parameters which indicate the tendency of the change in charge of ions with respect to the individual gas species and are each hereinafter called a pressure compensation factor.
In a related ion implanter, in order to simplify the situation, the following two types of gases are assumed to be present.
(a) plasma shower gas, such as Ar or Xe, used for plasma shower introduced for reducing charge-up caused by the ion beam.
(b) photoresist outgas, such as H2, CO2, or CH4, evolved by ion collision from a photoresist mask applied to wafers for forming a circuit pattern.
Accordingly, the equation (6) can be simplified as represented by the following equation (8).                                                                                                          f                  ⁡                                      (                    P                    )                                                  =                                                                            exp                      ⁡                                              (                                                                              -                                                          K                              A                                                                                ·                                                      P                            A                                                                          )                                                              ·                                          exp                      ⁡                                              [                                                                              -                            K                                                    ·                                                      (                                                                                          P                                AL                                                            -                                                              P                                A                                                                                      )                                                                          ]                                                                                                                                                                                                                      =                                  exp                  ⁡                                      [                                                                                            -                                                      K                            A                                                                          ·                                                  P                          A                                                                    -                                              K                        ·                                                  (                                                                                    P                              AL                                                        -                                                          P                              A                                                                                )                                                                                      ]                                                                                                          (        8        )            
In the above equation (8), PA indicates a partial pressure of a plasma shower gas, and PAL indicates a total pressure. (PAL−PA) indicates a partial pressure of a photoresist outgas obtained from the difference between the total pressure and the partial pressure of the plasma shower gas. In addition, KA indicates a pressure compensation factor for the plasma shower gas, and K indicates a pressure compensation factor for the photoresist outgas.
In ion implantation, since a plasma shower gas flow (typically 3 sccm of Ar) is controlled constant by the use of a massflow controller, the partial pressure PA of the plasma shower gas is measured right before the implantation and is assumed to be constant thereafter.
In addition, the partial pressure (PAL−PA) of the photoresist outgas varies with time due to the change in position of ion beam irradiation, depletion of the photoresist outgas, and the like one (see FIG. 2A), and hence the total pressure PAL is measured at regular time intervals during ion implantation, and the measurement is fed back to the Pcomp mechanism.
An example of calculating the pressure compensation factor KA for the plasma shower gas will be described with reference to FIG. 1. In this case, ion implantation is performed for bare wafers using Ar as the plasma shower gas at flow rates of 1, 3, and 5 (sccm). As the Ar flow is increased, the decrease in measured beam current Im is observed. In this case, since the photoresist outgas is not present (PAL=PA), from the equations (5) and (8), the following equation (9-1) can be obtained.                                                                         I                m                            =                                                I                  0                                ·                                  f                  ⁡                                      (                    P                    )                                                                                                                          =                                                I                  0                                ·                                  exp                  ⁡                                      [                                                                                            -                                                      K                            A                                                                          ·                                                  P                          A                                                                    -                                              K                        ·                                                  (                                                                                    P                              AL                                                        -                                                          P                              A                                                                                )                                                                                      ]                                                                                                                          =                                                I                  0                                ·                                  exp                  ⁡                                      (                                                                  -                                                  K                          A                                                                    ·                                              P                        A                                                              )                                                                                                          (                  9          ⁢                      -                    ⁢          1                )            
Accordingly, the following equation (9-2) is obtained, and KA is calculated from the slope of a fitting linear line shown in FIG. 1.In Im=In I0+(−KA·PA)  (9-2)
In the above equation, In indicates the natural logarithm.
An example will be described with reference to FIGS. 2A to 2C in which ion implantation is performed for wafers provided with a photoresist to obtain the pressure compensation factor K for a photoresist outgas. FIG. 2A shows the time dependence of the total pressure PAL in the vicinity of a position at which the wafers are irradiated with the ion beam, and FIG. 2B shows the time dependence of the measured beam current Im. The increase of the total pressure PAL is caused by outgas evolved from the photoresist. In addition, the periodic change in total pressure PAL is caused by the change in position of the wafers irradiated with ion beam, that is, the irradiation is started, for example, from the bottom end of the wafers and is returned thereto through the center, the top end, and the center of the wafers and is repeated in this manner. The reason the change in total pressure PAL is decreased with time is due to the depletion of the photoresist outgas. The trends of the measured beam current Im and the total pressure PAL with time are opposite to each other, and the decrease in measured beam current Im is observed when the total pressure PAL is increased.
FIG. 2C shows the relationship between the total pressure PAL and the natural logarithm of the measured beam current Im, and those can be plotted along an approximately linear line. From the equations (5) and (8), the following equation (10) is obtained.                               I          m                =                              I            0                    ·                      f            ⁡                          (              P              )                                                              =                              I            0                    ·                      exp            ⁡                          [                                                                    -                                          K                      A                                                        ·                                      P                    A                                                  -                                  K                  ·                                      (                                                                  P                        AL                                            -                                              P                        A                                                              )                                                              ]                                          
Accordingly, the following equation holds.                                                                         ln                ⁢                                                                  ⁢                                  I                  m                                            =                                                ln                  ⁢                                                                          ⁢                                      I                    0                                                  +                                  [                                                                                    -                                                  K                          A                                                                    ·                                              P                        A                                                              -                                          K                      ·                                              (                                                                              P                            AL                                                    -                                                      P                            A                                                                          )                                                                              ]                                                                                                        =                                                ln                  ⁢                                                                          ⁢                                      I                    0                                                  +                                  [                                                                                    (                                                  K                          -                                                      K                            A                                                                          )                                            ·                                              P                        A                                                              -                                          K                      ·                                              P                        AL                                                                              ]                                                                                        (        10        )            
Since the Ar flow is constant, the partial pressure PA is constant, and from the slope of the fitting linear line shown in FIG. 2C, the pressure compensation factor K can be calculated.
According to the related Pcomp mechanism, as described above, based on the assumption that the real beam current I0 is constant, the pressure compensation factors KA and K are obtained from the preliminary ion implantation. Subsequently, in actual ion implantation, by using these pressure compensation factors KA and K, the correction has been performed. A beam current IPcomp used for the control in actual ion implantation is represented by the following equation (11)                                                                         I                Pcomp                            =                                                I                  m                                /                                  f                  ⁡                                      (                    P                    )                                                                                                                          =                                                I                  m                                ·                                  exp                  ⁡                                      [                                                                                            K                          A                                                ·                                                  P                          A                                                                    +                                              K                        ·                                                  (                                                                                    P                              AL                                                        -                                                          P                              A                                                                                )                                                                                      ]                                                                                                          (        11        )            
When the beam current IPcomp is constant (that is, IPcomp=I0 holds), the equation (11) is equal to the equation (10).
However, in the related ion implanter, the following two problems have occurred.
The first problem is an underdose (deficiency of dosage) phenomenon generated when the pressure in the vicinity of wafers is considerably increased. The considerable increase in pressure in the vicinity of the wafers indicates that the amount of photoresist outgas is very large. The reason for this underdose phenomenon is believed that even though the real beam current I0 is assumed to be constant when the f(P) is calculated, the real beam current I0 itself is actually decreased.
The second problem is that when ion implantation is performed under various conditions, the loads of calculation of the pressure compensation factors KA and K are increased badly. That is, since the function f(P) is changed whenever the implantation condition, such as ion species and energy, is changed, the function f(P) must be re-measured for each implantation condition. In addition, even the implantation condition is not changed, since the individual variation among ion implanters is present, the function f(P) must be calculated for each ion implanter.